Synaptic transmission is the mechanism by which neurons communicate with each other. Synapse forms the link between neurons in a neuronal network and its plasticity certainly would have a profound effect on the network and animal behavior. However, this basic cellular mechanism is still poorly understood at a molecular level. This grant proposes to investigate short term plasticities and characteristics of tonic transmitter release in the crab T-fiber synapse. In the nervous systems of invertebrates and vertebrates, tonic synaptic transmission is associated with graded presynaptic depolarizations. The most notable example of tonic synapses in vertebrates is that between photoreceptors and secondary neurons in retina. The presence of dendrodendritic synapses have been reported in olfactory bulb, thalamus, motor cortex, and other areas of vertebrate central nervous system. Synaptic contact between dendrites presumably also have the potential of utilizing graded and tonic mode of transmitter release. The release processes of tonic synapses have not been investigated in depth mostly because of technical difficulties. The T-fiber preparation is technically advantageous for the investigation of tonic transmitter release because double electrode presynaptic penetration enables one to achieve a precise control of presynaptic membrane potential. In addition, this synapse expresses robust potentiation. The control of presynaptic potential also allows one to analyze potentiated release quantitatively. The proposed experiments are divided into three parts: (1) Quantitative characterization of tonic transmitter release, (2) Characterization of potentiated transmitter release, and (3) Modulation of tonic and enhanced transmitter release by second messenger systems.